As is well known, key telephone systems are used where a particular telephone station instrument or subset is to have the capability of direct access or connection to a plurality of lines or trunks included within the key system. In the older systems, this capability is attained by terminating all the line conductors for each of these lines directly at the station set and providing a key or button for each line. The particular lines to be used by the telephone station instrument is accessed by depressing the key for that line to directly connect the station instrument conductors to that line. An extra key is provided for placing a line in a hold condition when a second line is to be connected to the station instrument conductors to complete a call over the second line without disrupting the connection to the first line. Many other special services may also be provided, such as intercom, conference calling, priority levels, etc.
In the known systems, each line physically terminates directly at the station subset with its plurality of conductors so that as many as 50 conductors have to be cabled to a subset to control a standard six-button set, in which five buttons are utilized, one for each of five lines and the sixth button performs the hold control. For sets with greater number of line terminations, the number of conductors required may be particular multiples of the number of conductors required for a five line set.
In an attempt to reduce the number of these conductors, systems have been disclosed wherein only two speech conductors are actually connected to each station set, regardless of the number of buttons. In the known ones of these systems, at least four additional conductors are connected to the instrument for transmitting and receiving data to and from the station instrument to a line concentrator. Such systems frequently employ a central control which may be a data processor or computer, to cooperate with a concentrator in the form of a memory and a switching network to keep track of the states of the various lines and buttons and set up the connection through the network as required for key system operation.
Other known systems have been organized on a per line basis, with each line having two speech conductors, two data-in conductors and two data-out conductors. In one such known system, the conductors of each station have an appearance at one alignment on a crosspoint network, the trunks or lines having appearances along the other alignment. A control effects the switching of all conductors to complete a speech path and a signalling path through the crosspoint network.